elexis 3.x db cleanup

The following changes are planned for Elexis 2.3

• Use flyway to handle db-upgrades
• Use one flyway schema per plug-in which has a table. See Can I load flyway migration from a jar / multiple schemas
• handle choice of database connection before starting elexis (similar as what is realized in branch-2.2.dev)
• replace lastUpdate and other fields using custom methods to use proper datetime-columns
• replace extinfo with proper tables where apporiate, sa. also the thread ‘Zweites Elexis-Entwicklertreffen?’ in the Elexis-Forum